Home › Geophysical services › Seismic site classification — MASW and ReMi
Seismic / Code
Seismic site classification — MASW and ReMi
ASCE 7-22 assigns site class by shear-wave velocity alone. If your geotechnical report derived a class from blow counts, it no longer rests on an accepted basis.
What changed
For years a site class could be argued from a boring log — standard penetration resistance or undrained shear strength would do. ASCE 7-22 ended that. The six familiar classes became nine with the addition of BC, CD and DE, and the distinction between them is now based on shear-wave velocity alone.
Classes C through E are assigned from the average shear-wave velocity over the top 100 ft, and that average is a harmonic mean — soft near-surface layers weigh far more heavily than their thickness suggests. Classes A and B can only be assigned where velocity has been measured on site.
Where nobody measures, the default applies: a combination class taking the most critical spectral response across C, CD and D. It is deliberately conservative, because it has to cover the possibility that the ground is worse than anyone checked.
The nine site classes
Table 20.2-1 sets the boundaries by average shear-wave velocity over the upper 100 ft. The averaging is a harmonic mean, so a soft layer near the surface pulls the figure down much harder than its thickness alone suggests.
| Class | Description | V̄s (ft/s) | V̄s (m/s) |
|---|---|---|---|
| A | Hard rock | > 5,000 | > 1,524 |
| B | Rock | 3,000 – 5,000 | 914 – 1,524 |
| BC | Soft rock | 2,100 – 3,000 | 640 – 914 |
| C | Very dense sand or hard clay | 1,450 – 2,100 | 442 – 640 |
| CD | Dense sand or very stiff clay | 1,000 – 1,450 | 305 – 442 |
| D | Medium dense sand or stiff clay | 700 – 1,000 | 213 – 305 |
| DE | Loose sand or medium stiff clay | 500 – 700 | 152 – 213 |
| E | Very loose sand or soft clay | < 500 | < 152 |
| F | Soils requiring site response analysis | — | — |
Site Class F is not a velocity band at all. It covers ground that requires a site-specific response analysis — liquefiable soils, quick and highly sensitive clays, collapsible soils, peat and very thick soft clay. No measured velocity exempts a site from that determination, which is why a velocity survey answers the site class question but does not answer every geotechnical one.
Why it has become a data center problem
Large-footprint, fast-track projects feel this hardest. Seismic design governs a great deal of structure on a big slab, the schedule leaves no room to discover a problem late, and the permit set needs a defensible number rather than an assumption. Warehousing, manufacturing, healthcare and utility-scale energy are running into the same requirement.
Two methods, run together
- MASW — multichannel analysis of surface waves. An active source, with Rayleigh-wave dispersion inverted to a velocity profile. Strong resolution through the shallow section.
- ReMi — refraction microtremor. Passive, using ambient traffic and plant noise as the source, so it works on live industrial sites and reaches deeper than an active source alone.
Run together they extend the dispersion curve at both ends and produce a better-constrained profile than either alone. Neither requires a borehole, coring, or anything left in the ground. Most sites are a single day.
What you should ask for
A shear-wave velocity profile, the computed V̄s30, and the site class stated explicitly against ASCE 7-22 — in a report your structural engineer can put in the permit set. If a report hands you a class with no velocity profile behind it, ask how it was derived.
Whether a measured class reduces your design demand is the structural engineer's call, not ours. What measurement changes is that the decision rests on evidence rather than on the conservative assumption the code applies when nobody looked.
Equipment and capability
- MASW — active source surface wave
- ReMi — passive ambient noise
- Combined dispersion inversion
- V̄s30 harmonic mean over the top 100 ft
- ASCE 7-22 nine-class assignment
- Typically one day per site
What you receive
Shear-wave velocity profile with depth, dispersion curves and inversion misfit, the V̄s30 calculation shown rather than asserted, and the ASCE 7-22 site class stated in a form that goes straight into the permit set.
Ask before you commission
Tell us the target, the ground and the constraint and we will say whether this method answers it, whether it needs a second method alongside, or whether something else entirely is the better route. That conversation is free and it is usually short.